In this paper, we address the problem of testing hypotheses using
the likelihood ratio test statistic in nonidentifiable models, with application
to model selection in situations where the parametrization for the larger model
leads to nonidentifiability in the smaller model. We give two major
applications: the case where the number of populations has to be tested in a
mixture and the case of stationary ARMA$(p, q)$ processes where the order $(p,
q)$ has to be tested. We give the asymptotic distribution for the likelihood
ratio test statistic when testing the order of the model. In the case of order
selection for ARMAs, the asymptotic distribution is invariant with respect to
the parameters generating the process. A locally conic parametrization is a key
tool in deriving the limiting distributions; it allows one to discover the deep
similarity between the two problems.